A PALEOPROTEROZOIC ORIGIN OF OXYGENIC PHOTOSYNTHESIS AS A TRIGGER FOR THE MAKGANYENE SNOWBALL EARTH EVENT

While biomarker, isotopic, and trace element evidence have been used to claim that oxygenic photosynthesis evolved by 2.8 Ga (1, 2) and perhaps much earlier (3), the Archean evidence does not truly require the presence of oxygen and oxygen-producing bacteria. Persistent multiple oxygen indicators do not appear in the geologic record until the immediate prelude and aftermath of the ~2.3 Ga Makganyene Snowball Earth. This discrepancy supports an alternate hypothesis, in which the evolution of oxygenic photosynthesis triggered the geologically rapid destruction of a methane greenhouse and Earth's first global glaciation. New age constraints from the Transvaal Supergroup, South Africa, demonstrate that all three Huronian glaciations in Canada, which are unconstrained in latitude and may therefore be mid-latitude rather than global events (4), predate the Snowball event. A simple flux model of nutrient-limited cyanobacterial growth that incorporates the range of P and Fe fluxes expected during the Huronian glacial episodes indicates that cyanobacteria could have destroyed a methane greenhouse and triggered the Makganyene Snowball on timescales as short as 1 My. As the geological expression of oxygen does not appear during the ~2.8 Ga Pongola glaciation or during the Huronian glaciations, when glacial weathering should have elevated these fluxes, oxygenic cyanobacteria may not have evolved and radiated until shortly before the Makganyene Snowball. In addition to removing need to explain how PS-II could have been ‘bottled up' for 500-1500 My before the surface expression of oxygen became apparent, this scenario raises the specter of one mutant organism being able to destroy an entire planetary ecosystem: perhaps the first biogenic climate disaster.